6. Carbon dioxide

Carbon dioxide content in air is only 0.03%, but it is highly soluble in water unlike oxygen and one volume of CO₂ dissolves in equal volume of water, the solubility being higher at low temperature (see Table III). CO₂ stays in free (dissolved) or bound form (bicarborate and carbonate) in water depending on the pH of the water.

CO₂ + H₂O⇌H₂CO₃⇌H⁺ + HCO^-₃⇌H⁺ + CO⁼₃

The amount of unionized H₂CO₃ and free CO₂ can be estimated with the help of the Henderson - Hasselbach aquation, substituting 6.1 for pk, (changing with temperature) of H₂CO₃ thus:

at a pH of 7.4 the proportion of HCO₃^- to H₂CO₃ is 20:1; i.e.

antilog of 1.3 = 20, therefore - HCO₃^- : H₂CO₃ = 20.1

As obvious, low pH waters will have high dissolved CO₂ and water above pH 8.36, are free from dissolved CO₂. The relative levels of activity (mole fractions) of free and bound CO₂ (HCO₃^- and CO₃⁼) at different pH levels are shown in Fig. 11. Waters are classified by some authors (Birge & Juday in Welch, 1952) depending on the amount of bound CO₂: Soft waters - bound CO₂ level less than 5 ml/L (25 mgCaCO₃) (low calcium and magnesium); medium - bound CO₂ level, 5 – 22 ml/L; i.e. 100 mg CaCO₃; Hard waters bound CO₂ level over 22 ml/L. Hardness of waters will be dealt with further later.

Table. III: Solubility of CO₂ in pure water at differenct temperatures

Increase in free CO₂ is often concommittant with a decrease in oxygen in natural waters as CO₂ is an end product of anaerobic metabolism as well. Fishes in the tropics are often tolerant to high levels of free CO₂ in waters. Whereas the salmonids cannot tolerate CO₂ levels high than 5 – 10 mm Hg (pCO₂) the carps can tolerate CO₂ levels over 100 mm Hg. Again tolerance of a high free CO₂ level does not mean that the fish and shellfish are able to grow well under the high CO₂ condition. In waters into which sewage inflow is there the free CO₂ level can be high. In ponds with high photosynthetic activity the pH would go up due to the using up of the CO₂ by the plants and increase in the bound form (Fig. 12). The levels of pCO₂ should be ideally less than 5 – 10 mm Hg in fish ponds, especially if CO₂ sensitive fishes are being cultured.

Fig. 11. Mole fractions (%) of CO₂, HCO^-₃ and CO⁼₃ at different pH. (After Boyd, 1982)

Fig. 12. Diel change in pH in fish pond with moderate and low alkalinity waters
(After Boyd, 1979).

The biological role of CO₂ apart from being the end product of respiration and source for carbon fixation (photosynthesis) is that high levels of CO₂ interfere with the binding capacity of haemoglobin with oxygen. Shellfishes are also subject to the same effect even though their blood pigments are different (haemocyanin). The oxygen loading capacity is decreased at high pCO₂ levels and a shift of the blood oxygen (HbO₂) dissociation curve to the right is effected; the phenomenon is termed ‘Bohr effect’. In certain fishes as shown by Root (hence called Root effect) high pCO₂ reduced maximum blood oxygen capacity i.e. a depression of the oxygen dissociation curve is effected. Both together amplify the CO₂ effect on fishes, by restricting metabolism and energy release.